The IBM 3803/3420 Magnetic Tape Subsystem

The design innovations in the IBM 3803 Tape Control Unit and the IBM 3420 Magnetic Tape Unit are discussed. The new design concepts include a full readback parity check, a system of microprogram control that obtains high operating rates through overlap of channel control and motion control functions, a new phase-error detection system, a modified dual-density recording feature and a new interface between the control unit and tape drive. The combination of novel error detection methods, control features and programming support (including an expanded scheme of on-line diagnostics) contributes to greatly improved total system performance over previous models of tape subsystems. introduction Although half-inch magnetic tape stores a great deal of information per unit volume at a relatively low cost, it does present certain limitations, the greatest being that of the time interval required for data access. To overcome this limitation, it would be necessary either to increase tape speed, increase density, decrease head-gapldata distance, increase the number of parallel tracks, or, for certain applications, raise the rewind speed. To maintain compatibility with earlier IBM tape subsystems, the development of the IBM 3803/3420 magnetic tape subsystem was concentrated around improved access time and higher rewind speed. The IBM 3420 is the new tape drive and the IBM 3803 its control unit. Improved access time was achieved by positioning the readhead gap closer to the data, thereby reducing the access time interval in subsequent reads. Improved rewind time was achieved by more positive control over the tape as it enters the vacuum columns, and the control was obtained with a new configuration of tachometers for high-resolution tape speed information. The IBM 3803/3420 is a total replacement for any existing IBM 2803/2400 tape subsystem. The former now includes features called a two-channel switch, which allows connection to two separate channels, and device switching, which allows up to four control units to communicate with up to 16 tape drives. These two features, available in all combinations, have existed only on certain models in the past, and the device switching had required a separate unit. The general configuration is shown in Fig. 1 . This paper discusses the following new design features of the IBM 3803/3420 tape subsystem: Read-only storage control through microprogramming Phase-error detection and correction Full readback parity check Radial interface for increased function and serviceability Monolithic technology in the READ~WRITE channel New approach to reel control Greatly reduced dimensions of the control unit. General description A magnetic tape subsystem consists of an electronic tape control unit (designated in this paper by TCU), which connects one of several magnetic tape units (TU) to the computer’s central processing unit (CPU). The TCU, primarily an electronic unit, controls the operation of the subsystem. Channel commands are received by the TCU and decoded so that the required command can be sent to the proper tape unit. When an operation is completed, the TCU responds to the channel with the appropriate indication of status. When the processor is to perform a magnetic tape function, e.g., READ, WRITE or CONTROL, the command is sent to the TCU. If the operation is to be a WRITE, the TCU starts the tape motion and requests the first information from the processor. After all the information has been received, the TCU stops the tape motion and signals the processor that the operation is completed. The performance goals of the IBM 3803 Control Unit introduced some unique design problems. Early in the SEPTEMBER 1971 MAGNETIC TAPE E 391